Instruments which measure the concentration of a component of a sample gas often operate by measuring the absorption of infrared radiation passing through the sample gas. In particular, such instruments direct a flow of the sample gas through a sample cell having infrared transparent windows and irradiate that cell with infrared radiation. Since the component of interest will absorb infrared radiation at characteristic ones of the wavelengths emitted by the source, a fraction of the incident radiation will be absorbed thereby, the fraction depending upon the concentration of the component of interest. By measuring the quantity of radiation that is transmitted through the sample cell with, for example, a thermistor or Luft-type detector, the quantity of infrared radiation that is absorbed by the gas in the sample cell, and therefore the concentration of the component of interest, may be determined.
In order to provide an infrared intensity reference for such measurements, it is customary to include in the instrument a reference cell which is irradiated by a separate infrared source. Because the reference cell and source are similar to the sample cell and source, except for the fact that the reference cell does not contain any of the component of interest, the level of radiation transmitted through the reference cell provides a convenient reference or standard against which to compare the level of radiation transmitted through the sample cell. This referencing of the output of the sample cell to the output of the reference cell is usually accomplished either by comparing the outputs of two similar detectors which are positioned at the output ends of the two cells, or by providing a differential detector the output of which varies in accordance with the difference between the outputs of the two cells.
In instruments of the above-described type, a chopper disc is usually placed between the sample and reference cells and their respective infrared sources to modulate the intensity of the applied radiation. By rotating this chopper disc at a frequency of, for example, 10 Hz, a 10 Hz signal component is introduced into the output signals of the infrared detectors. By then passing the modulated detector output signals through a 10 Hz band pass filter, much of the high and low frequency noise that is present in those signals is eliminated. One instrument of the above-described type is shown in U.S. Pat. No. 3,729,264, issued on Apr. 24, 1973 in the name of Simazaki et al.
In spite of their advantages, instruments which use the above-described dual-cell, dual-source arrangement have several shortcomings which limit their performance and make them inconvenient to use. One of these shortcomings is that the intensity of the radiation from the two sources may not be the same. As a result, the output of the reference cell detector may cause the output of the sample cell detector to appear to be larger or smaller than is actually the case. Even infrared sources which originally provide infrared radiation of equal intensities may not continue to do so with the passage of time.
Another shortcoming of instruments which use the above-described dual-cell, dual-source arrangement is that they require delicate adjustments of the alignment between the infrared sources and the respective cells. In a typical instrument each infrared source is provided with two horizontal adjustment screws for aligning it with the respective cell and a third adjustment screw for adjusting the distance between the source and the cell. Since adjustments of this type are provided for each source, a total of six adjustments may be necessary.
In order to avoid these shortcomings, as well as the cost of providing dual infrared sources, some instruments have been constructed with a single source the output of which is divided among the sample and reference cells by a plurality of passages which converge in the vicinity of the source. One instrument of the latter type is described in U.S. Pat. No. 2,754,424, issued on July 10, 1956 in the name of Woodhull et al. While instruments of the latter type may not have the same short-comings as instruments of the dual-cell, dual-source type, they have disadvantages of their own which limit their utility. One of these is the overall reduction in the intensity of the infrared radiation for all cells that results from the multiple reflections within the various passages. A related disadvantage is the difficulty of providing a plurality of beams of equal intensities after the beams have been transmitted through passages which have differing lengths and differing reflective properties. Such a reduction in infrared intensity has the effect of worsening the signal-to-noise ratio of the instrument and therefore limiting the range at which accurate measurements may be made.
Another disadvantage of utilizing the above passages is the fact that the light that emerges from these passages is poorly collimated. Unlike the dual-source instruments in which parabolic reflectors can be provided for each source to collimate the beams emitted thereby, the radiation emitted by the single source through beam dividing passages is emitted at random angles. As a result, a sizable fraction of the source radiation is reflected from the surfaces of the cell windows, thereby further reducing the intensity of the radiation passing through the cells and further worsening the signal-to-noise ratio of the instrument.